In addition to conventional soldering using a lead-containing solder, electrically conductive adhesive bonding is increasingly being used to produce electrical connections between silicon components and carrier elements in the field of microelectronics. The main reason for this is the ban of the use of lead for soldering purposes decreed by the European Union. For this reason, it is necessary to use lead-free solders to produce electrical connections. However, the drawback of lead-free solders lies in the considerably higher melting temperature (170-185° C.).
For applications that require a low-heat joining process at temperatures up to at most approx. 85° C., electrically conductive adhesive bonding is the preferred connection technology.
In one specific example, an LED (light emitting diode) is to be mounted on a silicon carrier (submount). This requires the LED to be mounted on the surface of the silicon carrier and, at the same time, to be electrically contact-connected. To achieve this, the rear side contact of the LED has to be connected to the submount surface via a conducting connection. The second contact required is then contact-connected to the submount surface from the top side of the LED via a bonding wire. This connection can be produced with the aid of standard wire bonding techniques. For this purpose, aluminum contact surfaces are usually provided on the submount in order to allow contact-connection of the bonding wire, which may consist of aluminum.
However, on account of the inevitable formation of an oxide layer on the metal surface, the metallization (Al) used here is not suitable for electrically conductive adhesive bonding. The oxide layer (insulator), which is present on the metal surface, is formed in the case of components that are not hermetically packaged unless measures are taken to avoid this. This oxide layer forms because the epoxy resin adhesive that is customarily used is relatively pervious to moisture.
In order nevertheless to be able to realize an electrically conductive connection, an adhesive and conductive connection is required on what is known as the chip land surface.
Precious metals, such as silver, platinum, palladium, etc., have proven to be materials that can be used for this purpose, since they scarcely form an oxide layer or, in the case of silver, form a conducting oxide layer. However, on account of the electrochemical series, in conjunction with catalysts (e.g., moisture), aluminum pitting (corrosion) can occur.
Another possible option is to use ITO (indium tin oxide). A layer of this material is frequently used in semiconductor fabrication and in LCDs (liquid crystal displays) since it is both transparent and conductive. Therefore, this layer can be used as a light-transmitting electrode for LEDs (light emitting diodes) and also for LCDs. It can also be used as an opaque screening means, for example, above the photodiode of an optocoupler in order to increase the common-mode rejection or in very general terms to improve the sensitivity to interfering electromagnetic radiation.
If an ITO layer of this type is to be bonded to a standard aluminum metallization, the problem arises that the oxygen of the ITO layer oxidizes the uppermost Al layer, with the result that the inherently conductive connection is interrupted. To avoid this, a precious metal is recommended as an interface, in which context platinum can be used together with Ti or Cr as an adhesion promoter.
There are usually two ways of realizing a bondable and adhesive metallization. The first known way is a base metallization for bonding (e.g., Al) and a further precious metal metallization or a metal that is less prone to oxidation (e.g., TiPt), which is applied only in the region of the adhesive surface. The second way consists in a precious metal metallization, which is both adhesive and bondable (e.g., TiPtAu).
The first way has the drawback that corrosion can occur between the two metallizations via the intervention of a catalyst. The second way has the particular disadvantage that on account of the extremely low surface roughness of Au, bonding problems can occur with further layers or, for example, with potting resins. Furthermore, TiPtAu is available in very few silicon lines, and, consequently, additional investment costs would be incurred, whereas Al is a standard metallization for silicon.